The following is considered common knowedgle the citizens of the City of Greyhawk know this in some shape form or fashon.

The Castle was constructed CY c.320 by the wizard Zagig Yragerne. Known as "The Mad Archmage," Zagig ruled over the Free City of Greyhawk from the Castle for approximately the next 100 years, after which he abandoned the Castle and mysteriously disappeared.
Over the centuries, the Castle's three towers decayed into ruin. The main tower is called the "Tower of Zagig," and the two lesser towers the "War Tower" and the "Tower of Magic" (alternately "The Power Tower"). An extensive dungeon complex lies below the towers.
In the initial years after the Castle was abandoned, few if any adventurers approached the Castle because of rumors that it lay under a powerful curse. Eventually, a group of Northern barbarians forayed into the area and pillaged a large amount of treasure. The story of their success attracted groups of Dwarves, Elves and other adventurers to the area, and over time the Castle's dungeons became a major site for adventuring on the Flanaess. The influx of trade, travelers and treasure associated with this phenomenon has provided significant economic benefits to the nearby City of Greyhawk.
Sometime during this period, Zagig reappeared and managed to imprison the demon lord Fraz-Urb'luu in the dungeons beneath the castle. The fiend was imprisoned there for at least 200 years, but eventually freed by a wizard and a cleric (probably Erac's Cousin and his adventuring companion Ayelerach).
In CY 505, Zagig then captured and imprisoned the evil demigod Iuz in the dungeons beneath the Castle. Iuz was one of nine demigods so trapped, and this accomplishment was instrumental in Zagig's ascension to become the demigod now known as Zagyg. Other deities imprisoned in the dungeon included Merikka, Rudd and Wastri. The remaining five demigods have never been canonically identified. Because Zuoken is known to be imprisoned somewhere on the central Flanaess, some believe he was one of the nine deities Zagig trapped at the Castle.